The present invention relates generally to electric induction furnaces and more particularly to induction furnaces having improved efficiency coil systems.
Electric induction furnaces are used to heat and melt metals and other electrically conductive materials. An induction furnace utilizes an induction coil that is powered from an ac power source. Alternating current flowing through the coil creates a magnetic field that is applied to the electrically conductive charge placed inside of the furnace""s crucible. Eddy currents induced by the field in the charge can be used to heat, melt and superheat the charge. The magnetic coupling between the induction coil and charge is analogous to a magnetic transformer coupling. However an induction coil has a much higher leakage inductance than the leakage inductance of a magnetic transformer. Consequently an induction furnace""s power factor is extremely low, typically ranging from 0.08 to 0.15, lagging and, therefore, an extremely inefficient load.
The conventional coreless induction furnace consists of a copper water-cooled helical coil with a ceramic crucible containing the charge. Alternating current in the coil generates a magnetic field that induces current into the conductive charge. As illustrated in FIG. 1(a), the induction furnace 100 can be viewed as a loosely coupled transformer where the turns of the primary coil are magnetically coupled to a single turn formed by the conductive melt 102. In the figure, Ic represents the coil current and Im represents the current in the molten bath. Therefore, it can be assumed that the ratio of the current induced into the melt to the current in the coil approximates the number of coil turns. Maximum current density is induced on the circumference of the melt, with current exponentially decaying into the melt depth towards the center of the crucible.
The rate of decay is defined by a constant, namely the depth of current penetration into the metal, xcex94m, as defined (in meters) by the following equation:       Δ    m    =                              2          ·                      ρ            m                                                μ            O                    ·                      μ            m                    ·          f                      =          503      ⁢                                    ρ            m                    f                    
where: xcfx81m=resistivity of the molten metal (in ohms/m);
xcexcoxc2x7xcexcm=the product of absolute and relative permeability (with xcexco=4xcfx80xc3x9710xe2x88x927 and xcexcm, the relative permeability of the metal, in H/m); and
f=the frequency of the coil current (in Hertz).
Induction furnaces are usually designed to satisfy the condition that the depth of current penetration into the metal is much less than the radius of the melt (xcex94m less than  less than rm).
The conventional shape of the melt is cylindrical. Most of the induced current is flowing in the outer layer of the melt with thickness equal to the depth of penetration, xcex94m. The resistance, Rm, (in ohms) of this layer can be estimated by the following equation:       R    m    =                                          ρ            m                    ·          2                ⁢                  xe2x80x83                ⁢                  π          ·                      r            m                                                h          m                ·                  Δ          m                      =                  0.0125        ·                              r            m                                h            m                              ⁢                        f                      ρ            m                              
where
Rm=resistance of the melt (in ohms);
rm=radius of the melt;
hm=height of the melt;
xcfx81m, xcex94m and f are as previously defined.
Induction furnaces are principally single-phase devices. The supplied electric power is typically distributed over balanced three-phase lines. For optimal operation, induction furnaces operate at frequencies typically in the range of 100 to 10,000 Hertz. These frequencies are needed to maintain an optimal xcex94m/rm ratio for electromagnetic stirring of molten metal in the furnace.
Solid state power converters generate the power at required frequency, voltage and current for induction furnaces. These converters utilize power semiconductors (such as SCR, IGBT or IGCT topologies). The solid state static power converter resolves the phase balancing problem. Input 3-, 6-, or 12-phase line voltages are rectified before being inverted into a single-phase medium-frequency electrical current. Full-wave rectification of multi-phase line voltage produces a low harmonic distortion on feeding electrical lines, thus eliminating the need for line filters. As illustrated in FIG. 1(b), the power converter consists of three major sections:
an ac to dc rectifier and dc filter;
a dc to ac medium-frequency inverter; and
a bank of tuning capacitors.
Power supplied to the furnace is controlled automatically by varying the commutation timing of the inverter""s solid state switching components. This timing determines the operating frequency, phase and amplitude of the furnace current.
There are two conventional implementations of static solid-state power converters, namely a current-fed inverter with a parallel capacitor bank and a voltage-fed inverter with series capacitor bank. FIG. 2(a) illustrates a furnace system utilizing a current-fed converter. FIG. 2(b) illustrates a furnace system utilizing a current-fed converter with series/parallel tank capacitors. FIG. 2(c) and FIG. 2(d) illustrate furnace systems utilizing a voltage-fed converter in full bridge and half configurations, respectively. Each of these power supply topologies comprises a rectifier and filter section 110; a solid state inverter section 120; and a tuning capacitor section 130. While the generally recognized symbol for an SCR is used in these set of figures, other solid state switching devices can be utilized in these applications.
In the current-fed inverter, as illustrated in FIG. 2(a), the power factor correction capacitor bank is usually connected in parallel to the furnace coil. The term xe2x80x9ccapacitor bankxe2x80x9d is used here to designate one or more capacitors connected in series or parallel to be the equivalent circuit as shown in the figures. Both the capacitor bank and the coil are connected into the diagonal of a full-bridge inverter. This connection allows the reactive component of the coil current to bypass the inverter""s solid state switching components. However, the inverter is exposed to the full furnace voltage. The values of inverter voltage may be higher or lower than the dc voltage on the rectifier. Therefore, dc rectifier and inverter sections must be decoupled by reactors. The reactors supply the inverter with constant dc current. They are acting as a filter and reservoir of energy. The inverter converts dc current into square wave current that is supplied to a parallel resonant circuit.
The furnace power in current-fed inverter systems is controlled by varying both inverter timing and dc voltage. When inverter voltage falls below dc rectifier potential, the output power cannot be controlled by variation in inverter commutation frequency alone. Additional control of the injected dc current is carried out by regulating the conduction phase angle of the rectifier SCRs. Such regulation will introduce distortion into the feeding electrical line unless filters are provided.
The main advantage of the parallel resonant inverter is that only part of the coil current is passed via solid state switching devices, thus saving the number of semiconductor devices. The inverter controls only part of the coil current. This, however, limits the controllability of the inverter. Using the smoothing dc reactors as temporary energy accumulators causes difficulties in starting the inverters. The energy in the reactors is kinetic (analogous to the energy of a flywheel)xe2x80x94it exists only when the dc current flows from the rectifier to the inverter. To accumulate the necessary energy in the smoothing dc reactor, a special starter network is used. When the parallel inverter is stopped, the energy from this reactor is expended using the solid state switches of the inverter as a crow bar circuit.
The advantage of lower current in the inverter solid state switching devices is offset by a high voltage to which these devices are exposed. This often requires stacking the devices in series, which in turn necessitates special dynamic voltage dividers. For small current-fed inverters connected to standard low voltage lines, a series/parallel connection of capacitors is used, as illustrated in FIG. 2(b), rather than a parallel resonant circuit.
From the standpoint of electric circuit theory, voltage-fed series resonant inverters, as illustrated in FIG. 2(c), represent a duality circuit to the current-fed parallel converter. The current smoothing reactors in the dc line are replaced by dc voltage filter capacitors. The output parallel resonant circuit is replaced by a series resonant circuit. The voltage on the inverter is constant and equal to the output voltage of the ac to dc rectifier. The full coil current flows through the inverter""s SCRs and tuning capacitor bank. Such a configuration provides excellent controllability of the system. By controlling the timing of commutation of the inverter solid state switching devices, it is possible to rapidly change (within one oscillation period) the amount of energy circulating in the resonant circuit.
The potential electrical energy in the dc filter capacitor bank may be indefinitely maintained regardless of inverter status. During each cycle, the reactive power is flowing either from the filter to the furnace via the solid state switching devices or from the furnace to the filter via anti-parallel diodes.
Due to good controllability of the inverter section, there is no need to control dc voltage. Since phase control is not applied to the rectifier, the input power factor on the feeding line is relatively constant. No ac phase correction capacitors or line filters are required. Practical implementation of series resonant converters is even more simplified by utilizing a half-bridge inverter scheme as illustrated in FIG. 2(d).
A current-fed inverter operates with higher voltage, while a voltage-fed inverter operates with low voltage but full coil current. The voltage-fed inverter has better controllability and stores reactive energy entirely in the capacitors, which have lower losses than the dc reactors of the current-fed inverter. In all of these prior art configurations for induction furnace systems, the furnace coil is, as noted above, an extremely inefficient electrical load. Therefore, there exists the need for a higher efficiency coil system for an induction furnace.
In one aspect, the present invention is an apparatus for and method of heating and melting electrically conductive material in the crucible of an induction furnace system that includes a passive induction coil surrounding a partial section of the crucible. The passive induction coil is connected to a capacitor to for m an L-C tank circuit. An active induction coil, surrounding a partial section of the crucible is supplied ac current from an ac power source. The ac current generates a first magnetic field that heats and melts the electrically conductive material and, by magnetic coupling with the passive coil, induces an induced current in the passive coil. This induced current generates a second magnetic field that heats and melts the electrically conductive material. The resistance of the L-C tank circuit is reflected into the circuit of the active induction coil to increase the efficiency of the induction furnace system. These and other aspects of the invention are set forth in the specification and claims.